Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Tags

Stack, Design

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Array.prototype.shift

The shift() method removes the first element from an array and returns that element. This method changes the length of the array.

/**
 * @constructor
 */
var Queue = function () {
    this.stack = [];
};

/**
 * @param {number} x
 * @returns {void}
 */
Queue.prototype.push = function (x) {
    this.stack.push(x);
};

/**
 * @returns {void}
 */
Queue.prototype.pop = function () {
    this.stack.shift();
};

/**
 * @returns {number}
 */
Queue.prototype.peek = function () {
    return this.stack[0];
};

/**
 * @returns {boolean}
 */
Queue.prototype.empty = function () {
    return this.stack.length === 0;
};

/*
 * shift() 方法用于把数组的第一个元素从其中删除，并返回第一个元素的值。
 */